Doing an exploratory may be interesting, but may also not be necessary to correct your issue. Maybe your gas cap vent was plugged... A little more info on the condition would be helpful. Yes, flat head engines like this have been NLA for a few years now. OHVs will replace, but is expensive as you've found and require some modifications much of the time. Could you post a pic of the hole? There is a hole in the breather chamber by design. The question is, what is yours and will it affect anything if there is.

Why didn't I take a pic of the hole when I had the thing open? Will have to disassemble to take that pic. Will do that probably tomorrow.

Thank you!

tomplum, I just found a picture of a new B+S cylinder assembly, what is supposed to be a replacement for mine. Thought it's upside down, I can see that very same hole in the breather chamber! Whoo. What a relief. My whole looks more "busted out" where as the one in the pic is nice and round, but the size is about the same. So that's great! I can still post a pic of the hole if you like, but it sure looks like it's supposed to be there, like you said.

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Oh yeah, you mentioned a clogged gas cap vent. I loosened the gas cap and even removed it with no change (except that it splashed gas on me).

Simple as though it may be, what do you think of my theory about the missing engine bolts causing the drive belt to tighten and slacken reducing power to the wheels? Maybe my new bolts, along with a new head gasket, might do the trick. From my reading on this forum, maybe I should also check the breather and replace it's gasket. I'd like to check the piston rings, but access to the crankcase seems to only come from the bottom side of the engine. I'd rather not pull the engine from the chassis.

Thanks again!

Ok. I didn't take a photo of the breather chamber. That hole is rough, but it's in the same location and about the size as the one in the new engine block pictured above. So I think the hole is fine.

I did what I think is the correct procedure to test the breather. I could blow equally easily through each side. Is that good? It also looked clean.

Here are some pictures of the cylinder and valve heads (after cleaning). The main reason I took these is so you can see the scratches in the cylinder. They seem to be shallow, but are deep enough to feel with my fingernail. Acceptable? Thank you so much for your thoughts!

Oh, I've ordered new gaskets. When they come in later this week, along with a new breather (if you think I need one), I plan to put the mower back together and try it out.

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Ok, just to be clear (because sometimes I'm not!)

I have 3 questions:

1) Do I need a new breather?

2) Should I be concerned about those cylinder scratches?

3) Something else: I described the compression test I used from the B+S site. Turn flywheel counter-clockwise and see how fast it snaps back. Is this a decent test for compression. Mine quickly slowed to a stop, but I wouldn't say it snapped back.

Thanks!
Steve.

First, in response to above. 1)Possible. The breather tends to want to be a one way valve, but I think in how much mouth pressure you are using could vary the result. 2)There really isn't anything to do at this point other than being diligent about your clean up and getting the bits of Scotchbrite and carbon out. 3) Briggs hesitates to actually publish a spec for a basic compression test for good reason. But that isn't to say that an experienced tech won't ever put a compression gauge on a Briggs engine to do a ring test or use as a comparative figure. The flip it backward test works good enough, but sometimes is more difficult to implement. IE on a rider that one has a difficult time spinning the engine due to a belt dragging as example, or having a difficult time to get a good grip and room to spin etc. Works great on a walk behind mower where you can remove the plug wire, tie the brake open, use a glove and grab the blade and do the test. Since it is apart, gasket it up and give it a try. SHould you need more assistance with the runability issue,be more specific on what is happening. You seem to have a good attention to detail. Hopefully the vibration is gone too. If not....

tomplum, thanks again for all your help, especially for answering all 3 questions above.

When the gaskets come in, I'll put it all back together and give it a try. If it doesn't work, I'll try to give you more specifics about its behavior.

I doubt I'll order a new breather. The old one definitely had good air flow in one direction. Maybe I was just trying to hard to blow through the other way.

Thanks again!
Steve.

First the good news:
I put the mower back together last night and mowed a quarter of my yard tonight. The mower ran faster than it had before I bolted it properly to the chassis.

The not so good news:
It's still underpowered. It's slower than it used to be, and while it climbed a hill with no problem, it bogged down in thick grass.

Here's what I had done to "repair" it:
I bought new bolts to bolt it to the chassis (2 of 4 had fallen out). I bolted it on with lock washers and nuts. Seems to have held tightly tonight. I also inspected the valves, cleaned the combustion chamber, and replaced the head gasket, breather gasket, and intake gasket.

Here are my observations from tonight's experience:
1) Without the blade engaged, the engine sounds really rough. Engage the blade, and it gets pretty smooth.

2) I heard a constant squeaking. I remember some squeaking from the past, but it seems like more now. Loose drive belt?

3) When it bogged down in the thick grass (engine did not cut off), I disengaged the blade, and the mower just sat there for maybe 5 seconds before taking off with a jerk.

4) The mower shut off twice while I was mowing when it had gotten low on gas. The first time, I took the gas cap off, and I saw that gas was covering the fuel line opening, so it wasn't out of gas. I rocked the mower and saw some bubbles come up from the fuel line, and then it started. The second time, I still saw gas in the tank but couldn't get it started. It has always had a problem starting on a hot engine, so it isn't surprising it didn't restart. Previously on this forum, we diagnosed that problem as "tired starter". I never fixed that because it's not a big deal as long as the mower runs until I'm done each time.

Some thoughts:
1) I'm the 3rd owner of this beast. When I received it, the aftermarket muffle was hanging on by two non-factory bolts, and the bolt holes were pretty stripped out. The muffler fell off when the engine bolts dropped out and the engine vibrated so badly. I tried replacing it with a factory muffler, but I couldn't get those stripped bolt holes to work. So I replaced it with an aftermarket 12hp muffler, the kind that screws onto a pipe, and then the pipe screws into the engine block. Don't know what that might cause, but I thought I'd mention it. Here is the muffler. I used a 5" pipe. muffler link

2) The loss of power originally happened when the the 2 engine bolts fell out. The engine was vibrating very badly for a while, since I went ahead and finish mowing the grass (at least until the muffler fell off). So I think whatever happened (muffler, belts, etc) happened because of the shaking. Any thoughts of something that could have shaken loose during that time?

3) I'm thinking of doing a carb adjustment, especially after seeing my white exhaust valve. That means lean, right? What are the odds that a violently vibrating engine could knock the carburetor out of adjustment?

That's all I can think of at the moment. If you have any questions, please ask. I might have forgotten to list a symptom.

Thank you!
Steve.

There is no harm in richening the mix a tad if your carb has the adjustment screw below the bowl. If you haven't removed the bowl to give it a quick clean do so. You may need to have a look underneath at both belts plus the engine stack pulley itself. Murrays are notorious at twisting off the stamped key, so that pulley may be spinning on the crank when the going gets tough. Muffler set up seems ok.

Ok. Thanks, Tomplum!

I did clean the bowl earlier in the season, but a lot has happened since then. I'll clean it again and adjust the carb.

I did some research this afternoon regarding "the stamped key". I learned that this is kind of a tooth on the hole in the pulley. The key fits in a notch in the crankshaft so that the pulley is locked into the crankshaft. If that key comes off, the crankshaft can spin in the pulley hole. This becomes apparent when a load is placed on the engine. This sounds exactly like my problem! I'm willing to bet that when the engine was vibrating willy-nilly, thereby bouncing that pulley around, that key got sheared off. Providing that the crankshaft is not busted, I only have to replace the pulley, which should cost around $30.00 including shipping. I just wanted to post that summary (hopefully it's correct) to benefit anyone who has this problem in the future.

My immediate problem: how do I look under the mower? I thought I would buy a mower jack, but those are hundreds of dollars. If I drain the gas and oil, can I flip the mower on it's end (headlights pointed up)? Is there a better way to do this? Putting it up on jacks and blocks, and then crawling underneath, seems very dangerous. Any advice on this?

Tomplum, thank you again and again! Seems like my mower problem is almost fixed.

Steve.

Ok. Thanks, Tomplum!

I did clean the bowl earlier in the season, but a lot has happened since then. I'll clean it again and adjust the carb.

I did some research this afternoon regarding "the stamped key". I learned that this is kind of a tooth on the hole in the pulley. The key fits in a notch in the crankshaft so that the pulley is locked into the crankshaft. If that key comes off, the crankshaft can spin in the pulley hole. This becomes apparent when a load is placed on the engine. This sounds exactly like my problem! I'm willing to bet that when the engine was vibrating willy-nilly, thereby bouncing that pulley around, that key got sheared off. Providing that the crankshaft is not busted, I only have to replace the pulley, which should cost around $30.00 including shipping. I just wanted to post that summary (hopefully it's correct) to benefit anyone who has this problem in the future.

My immediate problem: how do I look under the mower? I thought I would buy a mower jack, but those are hundreds of dollars. If I drain the gas and oil, can I flip the mower on it's end (headlights pointed up)? Is there a better way to do this? Putting it up on jacks and blocks, and then crawling underneath, seems very dangerous. Any advice on this?

Tomplum, thank you again and again! Seems like my mower problem is almost fixed.

Steve.

If the engine bogs down in thick grass, the drive pulley is very likely OK. If the engine races when cutting thick grass, then the most likely cause is a slipping belt. You certainly could have a sheared pulley key, but that is not the most likely cause of your symptoms. Older gasoline engines are more likely to have carburetors with residual gum clogging up the works. Raising the mower with a jack, and resting it on blocks, is safe, provided that you are on a level concrete surface, and that your blocks are solid wood, such as a short section of railroad tie. You will need to identify and support the frame of the mower.

Thank you, ericwi.

That makes sense to me, the part about the engine bogging down vs. racing.

I don't think the engine bogs down in thick grass (though I don't remember it racing). The engine seems to continue full-speed, but the wheels stop turning.

I've been putting Sea Foam carb treatment in my gas all season. Hopefully the carb is pretty clean. Besides that, all this seemed to start when my engine bolts fell out and the engine vibrated like crazy. Makes me lean toward a mechanical problem more than a gummed-up problem.

I think I'll adjust the carb and then try the mower again, and this time, I'll pay extra attention to how it acts and sounds in thick grass.

Maybe I'll get a couple 8x8 beams to support the mower on if I need to get under it.

Thanks again for your help.
Steve.

The deck is really quite easy to remove and once that is off, you should have room to work.

Great. If it's that easy, I might as well get in there and start pullin' out bolts! Hope to try that tomorrow.

Thanks, guys.

Steve.

Full of good news tonight!

First, I didn't have to remove the deck. Just a cotter pin at the front of the deck to drop down the front piece, and then another cotter pin to drop one end of the tie rod. No problem accessing the pulley stack at that point, except that I needed to set the parking brake to loosen the drive belt.

Once that was done, the pulley stack fell off. The key was lodged in the crankshaft (I knocked it out with a screwdriver and hammer), and the top of the pulley stack is cracked. I've never been so happy to see a broken part in my life!

The crankshaft looks fine.

So I'm going to order a new pulley stack. I thought I'd be able to just order a single pulley, but now I see the stack is one big welded piece. It looks like it will cost around $70 including shipping, but I know this will fix my problem.

I also need to order the hex bolt, washer, and lockwasher that hold them on. Do you guys know of anything else I need in order to fix this? From the parts diagrams, it looks like this is everything.

Tomplum, thank you so much for your help on this! Looks like I'll get my mower working again for one last mowing before winter sets in. I really appreciate your patience with me.

Thank you!
Steve.

Cool beans!

Hi, Guys.

I ordered a new stack pulley from Northern Tool. The part that came in wasn't the Murray part I ordered but instead it was the Brigg's and Stratton part. Essentially the same thing. The pulleys were at slightly different heights than on the original, but were close enough. The good thing, though, is that this new stack pulley didn't have one key. It had 4 or 5. That should hold together much better. It was a tight fit on the crankshaft. I had to beat it on with a hammer (I hit the pipe, not the pulleys.) Then I put in the bolt and washers to hold it in.

And today I mowed! The mower only died once (engine and all). That was in some thick grass that was also a little wet. And the blade was at a low setting. I usually raise the blade when I mow that part of the yard, but today I forgot. Not a problem, though. I would have expected it to die even when the mower was at full health. The mower cranked back up and mowed fine once I raised the blade.

So it was successful in that I was able to complete the yard. The engine, muffler, and stack pulley all stayed in place. The mower still seems slow to me. My wife was watching, and she said it looked fine, but to me, it seems slow. I never did adjust the carburetor. I'm planning on doing that next to see if I can get a little more power.

Thanks.
Steve.

tomplum,
"...The breather tends to want to be a one way valve,..."
The breather IS a one way valve to stop air from going back into the engine.
You need the vacuum created for the motor to run smooth.

NewAtMowerRepair, FYI: The pic of the block only is a overhead valve eng block not your flat head - close but not the same but what your were looking at is the same- the oil passage to the valves and springs.

I think what your current fix is just a bandaid and you have a different problem.
If you just adjusted the carb because it was running lean, it would have run fasterand before stalling and burn out if lean enough. Remove the sparkplug if it is whiteish it is lean if goldish normal.

When u turn the crankshaft backwards and let it bounce back, turn it as fast as you can. if u turn it real slow, it won't bounce.

NewAtMowerRepair, try this - remove the airfilter. while it is idleing hold paper overthe air passage into the motor. does the paper "bounce" or is it just pulled down?
Look down into the passage to the carb. do u see a "puddle" of gas? or is it bone dry at the bottom?

If bounce and puddle,
Do u have a feeler gauge? If u take the plate off and look at the valve springs, turn the motor to Top Dead Center with both valves closed and measure the gap between the valves and the shaft holding each spring (tappet).

Outsider, thanks for all that information. I'll try that if the mower acts up again.

For now, though, I adjusted the carburetor last night. Don't have a tachometer, so I just set it back to factory conditions. One screw was 2.5 turns out from the seat, and the other was 2. I put them both at 1.5 turns. Seems to me the mower ran faster after I did that. If I'm still happy with it next time I mow, maybe over the weekend, I doubt I'll tweak it anymore. Seems good enough to get the job done in a reasonable amount of time.

Thanks again to you, tomplum, and to you, Outsider.

I'll update this post again if I do more with the mower.

Steve.